It is known that fuel cells allow electrical energy to be produced directly by an electrochemical oxidation-reduction reaction from hydrogen (the fuel) and oxygen (the oxidant), without conversion to mechanical energy taking place. This technology seems promising, especially for automotive applications. A fuel cell generally comprises a series of individual elements, each of which essentially consists of an anode and a cathode separated by a polymer membrane that allows ions to pass from the anode to the cathode.
As regards the fuel, either there is a hydrogen supply available or hydrogen is produced necessarily close to the fuel cell by means of a reformer which is itself fed for example with a hydrocarbon. As regards the oxidant, either the fuel cell is fed with compressed atmospheric air, and the excess gas in which the proportion of oxygen has decreased is discharged downstream of the fuel cell, or the fuel cell is fed with pure oxygen. The latter solution has a number of advantages, especially a more dynamic response of the cell to a current demand, this being particularly advantageous in the case of applications in transport vehicles, such as motor vehicles, which are known to impose particularly intermittent operating conditions, unlike in stationary applications. We should also mention, as advantages of a fuel cell being fed with pure oxygen, that the efficiency and power density are better and that there is no contamination by pollutants contained in the
However, in this case, the shut-down of the cell is not immediate as the system cannot profit from the asphyxiating effect of the nitrogen present in air. The electrochemical reaction cannot be completely interrupted by simply shutting off the fuel and oxidant feed valves. This is because the amount of oxygen and hydrogen remaining trapped within the respective channels of the fuel cell is sufficient to sustain the electrochemical reaction and there is a risk that this reaction will persist for several hours. Consequently, an electrical voltage persists across the terminals of the fuel cell to the detriment of safety. In the case of a fuel cell of the PEFC (Polymer Electrolyte Fuel Cell) type, maintaining a voltage without external current consumption (i.e. an open circuit voltage) involves mechanisms that cause the membrane to be rapidly degraded.
Patent Application WO 2005/088756 A1 describes a procedure for detecting leaks by measuring a change in pressure during shut-down of the cell. The pressure of the system may be varied by means of a recycling pump.
Patent Application WO 2005/078845 A2 proposes a procedure called “catalyst degradation suppressing”, for extinguishing a cell operating with hydrogen and air. To limit the catalyst degradation mechanisms appearing at full potential, the cell is shut down within a minimum time by drawing a substantial current so as to cause a rapid drop in voltage. During the cut-off procedure, the air feed is interrupted, while the hydrogen feed is maintained and the pressure appears to be controlled by means of the current. The cut-off procedure is designed to prevent hydrogen underfeed. A resistance is used to consume the waste gas. This is a cut-off procedure for an air fuel cell, the cathode circuit is vented to atmosphere during cut-off and a hydrogen feed is maintained during cut-off.
Patent Application WO 2006/064893 describes another cut-off procedure for an air fuel cell. Firstly, the hydrogen feed is interrupted, then a current is applied and, when the pressure drops slightly below atmospheric pressure, the hydrogen feed is maintained so as to keep the pressure at this level. On the cathode side, the air feed is firstly maintained so as to allow dilution of any hydrogen leaks via the purge valve and then it is interrupted until the oxygen is consumed. Many arrangements are provided for preventing hydrogen leaks at the valves. In the final embodiment presented, a complex calculation is proposed for maximizing the cut-off current as a function of the load capacity, the hydrogen concentration, the oxygen concentration and the voltage distribution. According to a comment on page 20, lines 22 to 26, all precautions are taken to prevent the pressure from dropping below a certain threshold as, according to this teaching, this would be prejudicial to the ion-exchange membrane.
Patent Application DE 100 59 393 describes a cut-off method for a fuel cell fed with pure hydrogen and pure oxygen. This Patent Application describes the following sequence: firstly the oxygen feed is cut off and then a variable electrical load is used to draw a current that causes the hydrogen/oxygen reaction to continue in the fuel cell. Next, when the oxygen pressure has dropped below a predetermined threshold, the hydrogen and oxygen circuits are flushed with nitrogen down to a predetermined pressure. This causes the fuel cell to shut down. However, this solution requires a reserve of nitrogen being available. Furthermore, the subsequent start-up of the fuel cell is ineluctably disturbed by the presence of nitrogen in the gas circuits.
Patent Application WO 2006/012954 also proposes a method of cutting off a fuel cell fed with pure hydrogen and pure oxygen. Unlike Patent Application DE 100 59 393, Patent Application WO 2006/012954 proposes at the end of the cut-off phase to flush the cathode circuit with atmospheric air (and not with inert gas), thereby benefiting from the asphyxiating effect of the nitrogen in air without having to have pure nitrogen and proposes to regulate the anode circuit so as to bring the hydrogen pressure gradually down to a level close to atmospheric pressure.
Although this solution does effectively achieve rapid and controlled shut-down of the fuel cell, without having to have recourse to a nitrogen feed, the shut-down cell is in a configuration such that some water remains, especially in the cathode circuit, which, a problem well known in fuel cells, makes it sensitive to icing and makes start-up problematic at a temperature below 0° C.
U.S. Pat. No. 6,068,942 proposes to cut off a fuel cell, fed either with oxygen or with ambient air delivering oxygen, by firstly interrupting the oxygen feed and then, when the oxygen partial pressure is below 0.5 bar, the hydrogen feed is interrupted. U.S. Pat. No. 4,226,919 proposes an arrangement of pneumatic valves for ensuring that: i) the hydrogen is introduced before the oxygen in start-up phase; ii) the oxygen is interrupted before the hydrogen in cut-off phase; iii) the oxygen feed is automatically interrupted should the hydrogen pressure drop during operation; and iv) the two circuits are flushed with hydrogen at rest.
Patent Application US 2001/0055707 proposes a system for purging the oxygen and hydrogen circuits with nitrogen during the cut-off phase so as to allow storage at low temperature (<0° C.).